Accurately measuring how many calories a person burns each day is difficult to do. Researchers can do this with a technique called doubly labeled water (DLW). This involves drinking water that is "labeled" with a non-radioactive tracer. After a few hours, the labeled water can be detected in the urine. To measure how many calories are burned (Total daily energy expenditure, TDEE), urine samples are collected several days apart. Although this technique is accurate, it is also challenging for two reasons. First, the labeled water is expensive. Second, the urine samples are analyzed using equipment (Isotope Ratio Mass Spectrometer, or IRMS) that is expensive and difficult to operate. The goal of this project is to develop a new instrument to perform DLW measurements of TDEE. This instrument, called a triple isotope water analyzer (TIWA) is less expensive and easier to operate than IRMS. Additionally, since the TIWA is more accurate than IRMS, it may potentially reduce the amount of labeled water required to measure TDEE, and thus reduce costs. The purpose of this study is to compare the accuracy of measuring TDEE from labeled water using the new instrument (TIWA) and from the traditional approach (IRMS). We will also compare the accuracy to the measurement of TDEE from whole-room indirect calorimetry (metabolic room), which is considered the most accurate way to measure TDEE.